1. Field of the Invention
The present invention relates to an optical pressure sensor equipped with an optical fiber including at least one FBG (Fiber Bragg Grating) portion.
2. Related Art
In the fields of design and analysis of automobiles, ships, airplanes and the like, various optimum designing has been carried out in terms of safety and higher efficiency. In particular, it is important to grasp the fluid physical quantity of air pressure or water pressure that acts on the object surface in the above fields. That is, optimum designing is carried out by feeding back the result obtained from the measurement of air pressure (wind pressure) by using a wind tunnel device that employs a reduced model and/or the measurement of water pressure by using a water tank.
A piezoelectric or semiconductor pressure sensor, and a strain gauge pressure sensor are known for measuring air pressure or water pressure.
However, such configuration has the following problems. In the piezoelectric and semiconductor pressure sensors, it is needed to bore a hole in the reduced model, insert a tube through the hole and electrically connect an opening end of the hole and a pressure sensor main body. The configuration has a technical limitation that the number of measurement points that could be arranged in the reduced model is up to tens to several tens. This is because the arrangement of a large number of measurement points is difficult since the tube has a diameter of several mm. With the strain gauge pressure sensor, an amplifier must be provided at each measurement point, and thus is practically difficult to carry out measurement at a large number of points. Furthermore, it is needed to bore a hole in an object body such as the reduced model in both the piezoelectric or semiconductor type and the strain gauge type. Therefore, the reduced model may be damaged due to water immersion from the hole when the reduced model is used for water pressure measurement. Furthermore, the strain gauge pressure sensor is electric, and therefore it may short-circuit due to water immersion.
An optical pressure sensor having an optical fiber with an FBG portion as a pressure sensitive portion is known as a pressure sensor that measures the pressure of the surface of the object body without boring any holes in the object body such as the reduced model (e.g., see Japanese unexamined publication No. 2002-71323, hereinafter referred to as prior document 1). The FBG portion is a portion where a grating (a diffraction grating) for reflecting only a predetermined wavelength of the incident light with respect to an advancing direction of the incident light is formed. The FBG portion is attached to a base film to form the sensor. When the pressure applying on the base film changes (i.e. when the base film strains due to the pressure change), the pressure change is also occurred on the FBG portion of the optical fiber attached to the base film. The pressure change on the FBG portion causes the diffraction pitch of the grating to change so that a peak of the wavelength of the light to be reflected is shifted. That is, the optical pressure sensor is configured so as to detect the amount of the pressure applying on the FBG portion (the strain amount of the FBG portion) by detecting the shift amount of the peak of the wavelength to be reflected at the time when the pressure applying on the FBG portion changes.
However, it is needed to detect a more minute pressure change in the optical pressure sensor as disclosed in the prior document 1 in order to carry out a safer and more efficient optimum designing. Further, there has been no optical pressure sensor capable of detecting both a positive pressure and a negative pressure (i.e., a differential pressure from a reference pressure).